Radio Frequency Identification (RFID) systems represent the next step in automatic identification techniques started by the familiar bar code schemes. Whereas bar code systems require line-of-sight (LOS) contact between a scanner and the bar code being identified, RFID techniques do not require LOS contact. This is a critical distinction because bar code systems often need manual intervention to ensure LOS contact between a bar code label and the bar code scanner. In sharp contrast, RFID systems eliminate the need for manual alignment between an RFID tag and an RFID reader or interrogator, thereby keeping labor costs at a minimum. In addition, bar code labels can become soiled in transit, rendering them unreadable. Because RFID tags are read using RF transmissions instead of optical transmissions, such soiling need not render RFID tags unreadable. Moreover, RFID tags may be written to in write-once or write-many fashions whereas once a bar code label has been printed further modifications are impossible. These advantages of RFID systems have resulted in the rapid growth of this technology despite the higher costs of RFID tags as compared to bar code labels.
Although RFID systems have certain advantages over bar coding schemes, they share many concerns as well. For example, bar code scanners can merely read a bar code label; they cannot provide a measure of quality. Because a marginal bar code may be readable by one scanner but not another, users have no way of reliably detecting the marginal bar codes using conventional bar code scanners. Thus, bar code verifiers have been developed to measure bar code quality metrics such as contrast, average bar deviation, and related quality indicia. Marginal bar code labels may thus be identified by bar code verifiers, thereby assuring users that their products may be reliably identified. The same concern for quality applies to RFID tags as well. However, the backscatter modulation commonly used to read information from passive RFID tags complicates the RFID verification process. In backscatter modulation, the interrogating RF beam itself provides the power for the RFID tag to respond. One verification metric would thus be how well a given RFID tag absorbed RF energy and retransmitted the energy to the RFID reader. But RF energy is absorbed by many objects in an RFID tag's environment. A conventional RFID reader has no way of determining whether a tag has absorbed RF energy or whether the absorption occurred due to environmental effects. Instead, a conventional RFID reader can merely determine the signal-to-noise ratio (SNR) of the backscattered signal from a passive RFID tag. A marginal RFID tag may be malfunctioning but illuminated with enough RF energy that the backscattered signal provided a sufficient SNR so that the RFID tag's signal may be decoded correctly. This same marginal RFID tag may be unreadable in less pristine RF environments. If an RFID tag could be verified to a known standard, such marginal RFID tags could be detected and replaced.
The need to verify RFID tags to a known standard is exacerbated by other RFID system properties. For example, RFID tags are not what-you-see-is-what-you-get (WYSIWYG) whereas a bar code label is. In other words, it doesn't matter what type of article a bar code label is affixed to because readability of the label is not affected, for example, by the article's color. However, the readability of an RFID tag may be strongly affected by the environment in which it is located. Thus, it is not possible to create a golden standard without knowledge of an RFID tag's context or environment. Moreover, because RFID tags can be physically or electrically damaged in transit, RFID systems are complicated by the need to find a safe position for the RFID tag. The juggling of RFID tag placement with RF absorption from the tag's environment can be a formidable task. Finally, the programmability of RFID tags requires that the fidelity of the RF link between an RFID reader and the RFID tag being interrogated must be relatively flawless. Accordingly, there is a need in the art to provide an RFID verifier that can more accurately verify operation of RFID tags using context-sensitive quality standards.